feat: 生图模型可选(火山/gpt-image) + 工作台图「加入资产库」收纳 + 任务中心排序修复

- AI 工作室生图支持显式选模型:resolve_image_model 解析 volcano(Seedream 图生图,
  无 image_edit 时走 image_generation 带参考图)/ gpt-image(image_edit 多图编辑),
  未选回落系统默认;enqueue_standalone_images 透传 image_model
- 资产库收纳:Asset 加 in_library 字段(迁移 0007,既有资产 db_default=True 不动);
  工作台生成图默认 in_library=False,只在工作台展示,用户「加入资产库」后才进库列表;
  assets 视图/序列化器/library 页/types 配套
- 任务中心修复:AITaskViewSet 默认 order_by(-created_at),前端 aiTasks 取 page_size=200。
  原因:列表无排序时 MySQL 按 UUID 主键乱序返回,把一批旧失败记录顶到首页,
  前端又只取首页 20 条算 全部/已完成/失败 → 误显示「全部失败」(实为 421 成功/少量失败)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/backend/apps/assets/serializers.py

@@ -87,6 +87,7 @@ class AssetSerializer(serializers.ModelSerializer):
             "description",
             "metadata",
             "is_deleted",
+            "in_library",
             "origin_task",
             "product",
             "files",
@@ -95,7 +96,8 @@ class AssetSerializer(serializers.ModelSerializer):
             "created_at",
             "updated_at",
         ]
-        read_only_fields = ["id", "review_status", "review_error", "created_at", "updated_at"]
+        # in_library 只读:增删资产库走专用 set-library 接口,不允许普通 PATCH 直接改
+        read_only_fields = ["id", "in_library", "review_status", "review_error", "created_at", "updated_at"]
 
 
 class AssetUploadSerializer(serializers.Serializer):